Laser lift off may be used to separate layers of material. One application in which laser lift off has been used advantageously is the separation of GaN layers from sapphire substrates when manufacturing light emitting diodes (LEDs). In spite of the advantages from UV-laser lift-off, GaN LED manufacturing has been limited due to poor productivity caused by low process yield. The low yield is due in part to high residual stresses in a GaN-sapphire wafer, resulting from a Metal-Organic Chemical Vapor Deposit (MOCVD) process. The MOCVD process requires an activation temperature of over 600° C. As shown in FIG. 1A, GaN and InGaN layers 32 are deposited on a sapphire wafer 38 by the MOCVD process. Since there is substantial difference in coefficients of thermal expansion (CTE) between the GaN (5.59×10-6/° K) and the sapphire (7.50×10-6/° K) (see Table 1), high levels of residual stresses exist when the GaN/sapphire wafer cools down to ambient temperature from the high temperature of the MOCVD process, as shown in FIG. 1B. The residual stresses include compressive residual stresses 40 on the GaN and tensional residual stresses 42 on the sapphire.
TABLE 1Various material properties of GaN and sapphire.BandLatticeLatticeGapThermalConst. aConst. cDensityEnergyExpansionMaterialStructure(Å)(Å)(g/cm3)(eV)×10−6/° KSapphireHexagonal4.75812.9913.979.97.50GaNHexagonal3.1895.8156.13.35.59
When an incident laser pulse with sufficient energy hits a GaN/sapphire interface, the irradiation results in instantaneous debonding of the interface. Since the incident laser pulse has limited size (usually far less than 1 cm2), it creates only a small portion of the debonded or lifted-off interface. Since surroundings of the debonded area still have a high level of residual stress, it creates a concentration of stress at the bonded/debonded border, resulting in fractures at the border. This fracturing, associated with the residual stress, has been one of the obstacles of the UV-laser lift-off process.
Currently, there are different ways to perform laser lift-off processes on GaN/sapphire wafers. One method involves raster scanning of a Q-switched 355 nm Nd:YAG laser (see, e.g., M. K. Kelly, R. P. Vaudo, V. M. Phanse, L. Gorgens, O. Ambacher and M. Stutzmann, Japanese Journal of Applied Physics, vol. 38 p. L217, 1999). This lift-off process using a solid state laser is illustrated in FIG. 2A. Another method uses a 248 nm excimer laser (see, e.g., W. S. Wong, T. Sands, N. W. Cheung, M. Kneissl, D. P. Bour, P. Mei, L. T. Romano and N. M. Johnson, Applied Physics Letters, vol. 75 p. 1360, 1999). This lift-off process using an excimer laser is illustrated in FIG. 2B.
Both processes employ raster scanning, as shown in FIG. 3, which involves either translation of the laser beam 44 or the target of the GaN/sapphire wafer 46. A problem associated with the raster scanning method is that it requires overlapping exposures to cover the desired area, resulting in multiple exposures 48 for certain locations. In both of the above methods, the laser lift-off of GaN/sapphire is a single pulse process. The unnecessary multiple exposures in localized areas increase the potential for fracturing by inducing excessive stresses on the film.
As shown in FIG. 4, raster scanning also involves a scanning of the laser beam 44 from one end to the other, gradually separating the GaN/sapphire interface from one side to the other. This side-to-side relaxation of residual stresses causes large differences in the stress level at the interface 50 between the separated and un-separated regions, i.e., the interface between the scanned and the un-scanned area. The disparity in residual stress levels at the interface 50 increases the probability of propagation of Mode I and Mode II cracks. Although the illustrations in FIGS. 3 and 4 are based on a process using a solid state laser, raster scanning of an excimer laser will produce similar results.
Currently, a common size of sapphire wafers is two-inch diameter, but other sizes (e.g., three-inch and four-inch wafers) are also available for the hetero-epitaxial growth of GaN. For a GaN/sapphire wafer, the level of residual stresses varies in the wafer, and compressive and tensile residual stresses may exist together. The existence of the residual stresses may be observed by wafer warping or bowing. When a laser lift-off process relaxes a large area of a continuous GaN/sapphire interface, as described above, a severe strain gradient may be developed at the border between the debonded and the bonded interface. This strain gradient may cause extensive fracturing of the GaN layer.
When a target material is irradiated with an intense laser pulse, a shallow layer of the target material may be instantaneously vaporized into the high temperature and high pressure surface plasma. This phenomenon is called ablation. The plasma created by the ablation subsequently expands to surroundings. The expansion of the surface plasma may induce shock waves, which transfer impulses to the target material. The ablation may be confined in between two materials when the laser is directed through a transparent material placed over the target. During this confined ablation, the plasma trapped at the interface may create a larger magnitude of shock waves, enhancing impact pressures. The explosive shock waves from the confined ablation at the GaN/sapphire interface can cause not only separation of the GaN layer from the sapphire substrate but may also fracture the GaN layer near the laser beam spot (see, e.g., P. Peyre et. al., Journal of Laser Applications, vol. 8 pp. 135-141, 1996).
Another technique for laser lift off involves the use of near-field imaging techniques to image a beam spot at the interface between the layers being separated. FIG. 5 illustrates one example of a projection of a homogeneous beam by near-field imaging and shows a representative beam profile along the beam path. The raw beam from an excimer laser 120 has Gaussian distribution in short side and flat topped distribution in the long side. The beam homogenizer 122 (e.g., of multi-array configuration) makes the gradient raw beam profile into a square flat-topped profile. The homogenized beam is cropped by the mask 124 (e.g., the rectangular variable aperture) to utilize the best portion of the beam, which is projected to the LED target wafer 116 by near-field imaging, for example, using beam imaging lens 126. The edge resolution of the homogeneous beam spot 130 at the LED wafer 116 therefore becomes sharp.
FIGS. 6-8C illustrate one way in which the imaged beam (e.g., the beam spot 130) can cause separation of layers of material of the LED wafer 116. Referring to FIG. 6, the laser may be directed through at least one layer of substrate material 102 (e.g., sapphire) to at least one target material 104 (e.g., GaN) to separate the materials 102, 104. The separation of the materials 102, 104 may be achieved by using a laser energy density sufficient to induce a shock wave at the interface 106 of the target material 104 and the substrate material 102, thereby instantaneously debonding the target material 104 from the substrate material 102. The shock wave may be created by the explosive expansion of plasma 108 at the interface as a result of the increased density of the ionized vapor sharply elevating the plasma temperature. The laser energy density may be in a range sufficient to induce a force Fa on the target material 104 that causes separation without fracturing. The applied force Fa may be represented as follows:Pp(GPa)=C[Ir(GW/cm2)]1/2 Fa(N)=Pp(GPa)Ar(cm2)where Pp is the peak pressure induced by explosive shock waves, C is an efficiency and geometrical factor, Ir is the irradiance of the incident laser beam, Fa is the applied force and Ar is the area under irradiation.
When the plasma 108 is expanding, as shown in FIG. 7, the irradiation zone is acting as a bending arm pivoting at the edge of the irradiation zone. For example, the force (Fr) required for rupturing or fracturing may be viewed as a two-point bend test and may be represented as follows:
      F    r    ∝                    wd        2            L        ⁢          σ      r      where d is the thickness of the target material 104, w is the width of the applied force or width of the laser pulse, L is the length of applied arm or half length of the laser pulse, and σr is the modulus of rupture or fracture stress of GaN. To increase the force (Fr), the width w of the laser pulse may be increased and the half length L of the laser pulse may be decreased, thereby forming a line shaped beam. The line shaped beam may be scanned across the target material 104 to minimize the bending moment upon irradiation.
At a laser energy density under the ablation threshold of GaN (˜0.3 J/cm2 at 248 nm), for example, the instantaneous separation of the GaN/sapphire interface 106 may not be successfully achieved, as shown in FIG. 8A. Although decomposition of the GaN can occur under the ablation threshold, this alone cannot achieve instantaneous separation of the interface 106, because there is no driving force, i.e. shock waves from the expanding plasma, without the ablation. Conversely, applying overly-intense laser energy density may create excessive explosive stress wave propagation, which results in cracks and fractures on the target material 104 (e.g., the GaN film), as shown in FIG. 8C. When the irradiating laser energy density is optimized, as shown in FIG. 8B, the force created by the shock wave is sufficient to separate the layers 102, 104 at the interface 106 but not enough to induce fracture in the target material 104. According to this example with GaN and sapphire, the laser energy density may be between about 0.60 J/cm2 to 1.5 J/cm2 to achieve the separation shown in FIG. 8B.
The near-field imaging techniques described above have been used successfully in a process known as patterned laser lift off to overcome many of the problems associated with residual stress and other problems discussed above. The patterned laser lift off technique forms streets in one or more layers to be separated, forms a beam spot to cover one or more of the sections defined by the streets, and separates layers in the sections. One example of a patterned lift off method is described in greater detail in U.S. Pat. No. 7,202,141, which is fully incorporated herein by reference. Although successful, the patterned lift off technique requires the additional steps of forming the streets, which results in a more time-consuming process. Attempts at using the techniques described above to provide monolithic laser lift off, however, have been less successful because of the problems associated with residual stress.